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I Just Don't Get It




 
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KA8WTK
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« on: January 09, 2005, 02:37:06 PM »

I have been working on a Globe Champion 300. This is one I got about 2 years ago and replaced the bad AX9909's with a single 7094. At that time the rig would load right and it did not function badly at all. :-P

I went to use the rig the other night and it just ain't right. Into a dummy load, it will load properly but the output is lower than expected (275 ma. measured at the cathode for 135 watts out - 1000 volts on plate). Worse than that is when you hook-up to an antenna that on any other rig has an SWR of 1.2:1, the SWR appears to be over 6:1! :twisted:

The neutralization has been checked. Voltages and currents check. Subbed in a different blocking cap, tank coil, etc. but nothing changes. I am left with the option that the 7094 has taken some kind of internal damage, probably the screen although the current reads proper.

Anyone have any other ideas or seen this before? I just don't get it! I must be missing something, give me a hint what it could be. :oops:

Thanks!
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Bill KA8WTK
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« Reply #1 on: January 09, 2005, 02:45:18 PM »

Bill,
    What freq. were you tuning up on? Like me, I found out that the on 160M the tank will only goes as far as 300Ω. Try tuning on 75m and I'll bet you'll get max smoke. BTW, you wouldn't happen to have a spare LV transformer??? Mine went to max smoke and released it all into the basement.  :cry: After the fire department went home, I saw that the transformer had cooked and leaked the enamel onto my bench. Any help is greatly appreciated.
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KA8WTK
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« Reply #2 on: January 09, 2005, 03:41:38 PM »

I have not tried the rig on 160 because of the 300 ohm minimum limit for the tank. All testing has been on 75 as the dummy load has lowest SWR there and the 75 meter dipole works very well with all my other tube transmitters (link or pi output) on this frequency.

I am sorry, I don't have a transformer for you. If I did, you could have it!
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Bill KA8WTK
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« Reply #3 on: January 09, 2005, 05:07:58 PM »

Quote from: Bill
I have been working on a Globe Champion 300. This is one I got about 2 years ago and replaced the bad AX9909's with a single 7094. At that time the rig would load right and it did not function badly at all. :-P

I went to use the rig the other night and it just ain't right. Into a dummy load, it will load properly but the output is lower than expected (275 ma. measured at the cathode for 135 watts out - 1000 volts on plate). Worse than that is when you hook-up to an antenna that on any other rig has an SWR of 1.2:1, the SWR appears to be over 6:1! :twisted:

The neutralization has been checked. Voltages and currents check. Subbed in a different blocking cap, tank coil, etc. but nothing changes. I am left with the option that the 7094 has taken some kind of internal damage, probably the screen although the current reads proper.


Anyone have any other ideas or seen this before? I just don't get it! I must be missing something, give me a hint what it could be. :oops:

Thanks!

Almost sounds like it is acting like a doubler. Ck for a loud 2nd harmonic.
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KA8WTK
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« Reply #4 on: January 09, 2005, 08:05:38 PM »

Dave,
  Got here and read your post at 7:55 PM and gave that a shot. There is, in fact, a strong 2nd harmonic. And to confirm what was happening, I turned on the frequency counter and got a reading near the tank coil. The VFO is set for 3.885 and the counter reads 7.770 when picking up the stray RF by the amplifier. So, I moved away form the transmitter and went to the watt meter to see if I could pickup any stray RF there. Counter still reads 7.770 and that sure isn't what it shoud be!
 
Good idea! Now if we can figure out why..........

THANKS!
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Bill KA8WTK
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« Reply #5 on: January 09, 2005, 08:40:37 PM »

Good call, Dave!!

I ran into this problem with my Viqueen II.  If you don't start with the roller inductor at the lowest freq position and the tuning cap full closed, I found it's really easy to tune up on a harmonic!  Try tuning up by starting with all cariable caps full closed.  Also could be a switch section that's outta sync or not moving - or the knob slipped a few degrees!  *Both* of those happened on my Junkston Valiant.

73 John
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KA8WTK
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« Reply #6 on: January 09, 2005, 08:50:20 PM »

Just checked the various stages with the frequency counter. VFO, buffer/doubler and driver/doubler are all reading and delivering 3.885 to the grid of the 7094. The 7094 appears to be our "doubler".

A I posted earlier, I have checked the plate choke, blocking cap and tank circuit. Tomorrow (even though I want to tonight) I will pull the plate the socket is mounted on and check all the bypass caps under the socket. The GC-300 doesn't have the bottom of the socket exposed from the bottom of the chassis.

Also, did as you suggested JN and checked the switches. No Joy.

Good calls from both of you!

(I love this board!)
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Bill KA8WTK
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« Reply #7 on: January 10, 2005, 08:14:04 AM »

Update.....
You are both right! The GC was tuned to a harmonic and the plate tuning knob was installed 180 deg. from where it should be. I don't know how I managed to get it to work before. The output is still low, but with it tuned correctly it is time to re-check the neutralization and voltages.

I do find it odd that a commercial rig will tune the harmonic. None of my other commercial tube radios seem to be able to do that, but I am going to confirm that transmitter by transmitter just to find out for sure.

More later.........
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Bill KA8WTK
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« Reply #8 on: January 10, 2005, 10:05:37 AM »

Bill, you went about checking this exactly the right way.  Used to be you had to use an absorption wavemeter or a grid dipper to find harmonics.

Perhaps the low output as well as the sudden multiplication problem is related to a component changing values in the grid or tank circuit of the 7094.  Are you getting plenty of grid drive?

Good job!  73 John
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« Reply #9 on: January 10, 2005, 11:08:15 AM »

Thanks,

I can get way more grid drive than required for 7094. I have found two old QST articles on the ARRL web page for amps that use 7094 tubes. The only difference in component values that is of note is that they are using a 1000 mmfd blocking cap in the articles and the GC uses a 500 mmfd. The single tube amp article uses a R-175A plate choke, and this is what is in here. Even the pi network cap values are very close and the GC tank coil is the same diameter and very close in description. There may be some re-tapping of the coil that would be of benefit, but the caps should have plenty of leeway to compensate. It will dip and load.

So, I think the first thing tonight is still neutralization and a confirmation of screen voltage and current values. May try a change in blocking cap value to what the QST articles state. I don't recall ever seeing a "guide" on what that value should be.
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Bill KA8WTK
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« Reply #10 on: January 13, 2005, 07:22:37 AM »

The Apache was known for having two plate dips on 80/75.  Many a Novice back in the olden days got an OO slip or worse for a second harmonic above 7400.   That certainly explains your SWR problem, eh?
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